The present invention relates to an apparatus for measuring optical density and, more particularly to such apparatus which produces a direct analog voltage output.
Apparatus for measuring optical density may also be characterized as a spectrophotometer. In general, spectrophotometers of the present type include a light source which provides a reference signal and a sample signal and compares the two resultant signals with the discharge voltage of an RC circuit. The time lapse between the two compare indications corresponds to the density of the sample. Thereafter, as typically shown by Liston U.S. Pat. No. 3,664,744 and Bowker U.S. Pat. No. 3,528,749, a digital counting technique is utilized to determine the time interval between the sample and reference compare indications.
However, many times it is desirable to provide an analog signal readout which is directly proportional to optical density. A digital counter output can, of course, be converted to an analog voltage, but only with excessive added circuit complexity and cost.